The present invention relates to an image recording method and an image recording apparatus, each for recording an image on a recording medium by ejecting ink droplets from an inkjet head.
As a method of recording an image on a recording medium, there is provided an inkjet recording method in which ink droplets are ejected from an inkjet head to form an image.
The inkjet recording method has a problem in that, because ink droplets are ejected from a plurality of ejection ports, variation in ejection characteristic of each recording element provided with the ejection port causes density unevenness in a recorded image. This problem is particularly conspicuous in a case of a single-pass type inkjet method in which, with a line-type inkjet head being fixed, a recording medium is conveyed once in one direction, whereby an image is recorded on the entire surface of the recording medium.
As a method of correcting the density unevenness, there are provided a method in which the density unevenness is corrected by changing, for each recording element, an ejection driving condition in accordance with the density unevenness and adjusting the dot diameter or the dot density, and a method which eliminates the influence of the density unevenness on a recorded image by correcting image data in accordance with the density unevenness.
The correction method by changing the ejection driving condition is such a method that makes a change with respect to the ink droplets to be ejected from an inkjet head, and hence, at the time of implementation, there exists a limitation on the driving method of the inkjet head and the correction range. On the other hand, the method by correcting the image data in accordance with the density unevenness can be implemented by correcting data without changing the actual ink droplets to be ejected from the inkjet head, that is, without changing the inkjet head itself (that is, without physical change thereof). Therefore, this method has greater flexibility, and various types of such correction methods have been proposed.
Here, in the case of converting image data, γ conversion is performed for each recording element with the use of a 1D-LUT.
As a method for obtaining a correction curve (unevenness correction coefficient) of the 1D-LUT, there are proposed a method in which, as in JP 04-18356 A, the density of an area corresponding to the position of a recording element is measured to thereby correct the density unevenness of a print area, and a method in which, as in JP 2006-264069 A and JP 2006-347164 A, the accuracy of the position of a droplet ejected from a recording element is measured with high precision, and a correction coefficient is calculated based on the positional information.
Here, JP 04-18356 A describes a method in which ink droplets are ejected from all the recording elements to create, on a recording medium, a solid image having a given density (for example, density of 50%), and, based on density variation of the image, the density unevenness is calculated and then corrected. Further, JP 04-18356 A also describes creating, by calculating only the amount of change from the last density unevenness correction data, correction data in a shorter period of time compared to creating correction data again from the beginning.
Further, JP 2006-264069 A gives a description as follows. Ink droplets are ejected from each ejection nozzle to form a test pattern, in which lines are made by the respective ejection nozzles. After the test pattern is read, based on a density profile of each line included in the read test pattern, a landing position error of the ink droplets ejected from each nozzle is detected, and, based on the landing position error, the density unevenness is corrected. Further, in JP 2006-264069 A, there is a description that, at the time of detection of the landing position error, the error characteristic of an ejection amount from a nozzle may be detected.
Further, in JP 2006-347164 A, there is a description that, based on the detected landing position of an ink droplet, a correction coefficient is calculated.
Here, to attain high-precision print quality, a pixel density of, for example, 1,200 dpi or higher is required as the pixel density of the inkjet head. Accordingly, one droplet (impact point) becomes smaller, and hence an interval error between the impact points becomes extremely small as well.
Further, the method of measuring the image density of an area corresponding to the droplet landing position of each recording element, which is described in JP 04-18356 A, requires a resolution at least twice as high as the resolution of the image so that the correspondence between the position of the recording element and the measurement in the relevant area can be obtained with high precision.
Accordingly, in a case where the method described in JP 04-18356 A is used for correcting density unevenness of a high-pixel-density image as described above, a resolution of 2,400 dpi or higher is required. As a result, it takes an extremely long period of time to perform scanning, measurement data transfer, and measurement data processing.
Incidentally, with regard to the method in which the area density is measured, it is known that even such a method, in which reading during the measurement is performed in a low resolution so as to reduce the required period of time, and then the density of each recording element area is estimated, has the effect of correcting unevenness. However, if a scanning resolution is made lower, the effect of unevenness correction becomes insufficient for unevenness having high-frequency components higher than the scanning resolution.
In addition, the method described in JP 04-18356 A has a problem in that sufficient precision cannot be attained by performing the unevenness correction once.
Further, when used for the density unevenness correction of a high-pixel density image as described above, the methods described in JP 2006-264069 A and JP 2006-347164 A, too, require a resolution of 2,400 dpi or higher in order to measure a dot position with high precision, and have a problem in that it takes an extremely long period of time to perform the scanning, the measurement data transfer, and the measurement data processing.
Particularly, a method in which a dot position and a dot diameter are detected based on the density profile as in the method described in JP 2006-264069 A requires particularly high-precision measurement due to the need to calculate the outer shape and density of a dot accurately. Besides, there is a problem in that, because the calculation is performed for each recording element, the calculation amount becomes larger, and it takes a longer period of time for the data processing.
Further, with this method, another problem is that, in some cases, low-frequency unevenness is not sufficiently eliminated depending on the type of position error.